Advanced downlink beamforming techniques are particularly important in Third and Future Generation Cellular Communication Systems due to asymmetric services present in such communication systems.
The eight documents listed below are incorporated herein by reference, and are hereinafter referred to by the corresponding numerals in square brackets shown below.    [1] S. Anderson, B. Hagerman, H. Dam, U. Forssen, J. Karlsson, F. Kronestedt, S. Mazur, K. J. Molnar, Adaptive antennas for GSM and TDMA systems, IEEE Personal Communications, Vol. 6, No. 3, pp. 74–86, June 1999.    [2] Bo Goransson, Bo Hagerman, Jozsef Barta, Adaptive Antennas in WCDMA Systems—Link Level Simulation Results Based on Typical User Scenarios, IEEE VTC 2000 Fall, pp. 157–64, Vol. 1.    [3] R. A. Soni, R. M. Buchrer, R. D. Benning, Transmit Beamforming Combined With Diversity Techniques for CDMA 2000 Systems, ICASSP 20001.    [4] K. I. Pedersen and P. E. Mogensen, A Simple Downlink Antenna Array Algorithm Based on a Hybrid Scheme of Transmit Diversity and Conventional Beamforming.     [5] M. Katz, J. Ylitalo, Extension of Space-Time Coding to Beamforming WCDMA Base Stations, IEEE VTC 2000 Spring, pp. 1230–4 m, Vol. 2.    [6] A. Hottinen, O. Tirkkonen, R. Wichman, Closed-Loop Transmit Diversity Techniques for Multi-Element Transceivers, IEEE VTC 2000 Fall, pp. 70–3 vol. 1.    [7] Enhance the beamformer feature of the multiple antenna Tx Diversity, TSGR1 #15 (00)-1065.    [8] Description of the eigenbeamformer concept (update) and performance evaluation, TSGR1 #19 (01)-0203.
Steering a radiated beam in the wrong direction (the so called beam pointing error problem), or using the wrong beam in a fixed multi-beam antenna system to transmit data to a specific mobile has serious consequences to system capacity. A mobile terminal radiated by a beam that points in a wrong direction will request more power to satisfy its desired Quality of Service (QoS). Since the peak of the beam is pointing in a direction other than the intended user, many other mobiles falling within the beam will experience an increased level of interference. Consequently, these mobiles will request more power to meet their own QoS, quickly using up the scarce power resources of the communication system. FIG. 1 illustrates situations where beam pointing errors will occur.
Beamforming solutions that are currently in use or have previously been proposed are discussed below.
Adaptive antenna arrays have been used successfully in GSM and TDMA systems, as reported in [1]. The aim in an adaptive antenna array system is to replace the conventional sector antenna by two or more closely spaced antenna elements. The key idea in antenna array systems is to direct the radiated energy from the array to the specified user of interest, while at the same time the antenna array system seeks to minimize the interference to other users. Such strategies have been shown in GSM and TDMA systems to yield improved performance, measured in increased system capacity and/or increased range, as reported in [1]. In [2], results show that the performance gain obtained by an advanced antenna system could be substantial compared to an ordinary sector covering system. Broadly speaking, adaptive antenna systems are grouped into two categories: a) fixed-beam systems, where radiated energies are directed to a number of fixed directions; and b) adaptive systems, where the radiated energy is directed towards any desired location.
In Frequency Division Duplex (FDD) mode of communication, such as WCDMA-FDD, the uplink data (from mobile to base station) and the downlink data (from base station to mobile) are transmitted on different frequencies. The frequency gap between uplink and downlink almost surely ensures different fading channel realizations. Thus, while the angle of arrivals and angle of departures of the signals are reasonably the same, the channel attenuations on every uplink and downlink path are not.
In FDD systems the downlink beam is computed using uplink measurements. This strategy will yield beam pointing errors, especially when the spread of the angles of arrival is large (larger than the beamwidth). Beam pointing errors will also occur when the angular spread is small and the mean of the spread is located near the cross over region of a fixed multi-beam antenna system. See FIG. 1 for an illustration of this problem.
Time varying multipath fading seriously degrades the quality of the received signals in many wireless communication environments. One method that mitigates the effects of deep fades and provides reliable communications is the introduction of redundancy (diversity) in the transmitted signals. The added redundancy can take place in the temporal or the spatial domain. Temporal diversity is implemented using channel coding and interleaving, while spatial diversity is achieved by transmitting the signals on spatially separated antennas or using differently polarized antennas. Such strategies ensure independent fading on each antenna.
Spatial transmit diversity can be subdivided into closed loop or open loop transmit diversity modes, depending on whether or not feedback information is transmitted from the receiver back to the transmitter. The 3rd Generation Partnership Project, Release 99, mandates that all mobile user equipment must support transmit diversity using two transmit diversity antennas. One open loop mode and two closed loop modes must be supported for the downlink dedicated physical channels.
Open loop transmit diversity employs a space time block coding transmit diversity (STTD). The STTD encoding is applied on blocks of four consecutive channel bits. The mobile user equipment does not transmit any feedback information back to the transmitting diversity antennas.
In closed loop transmit diversity, the spread and scrambled signal is subject to phase (in Mode 1), or phase and amplitude (in Mode 2) adjustments prior to transmission on two antennas. The antenna weights are determined by the mobile user and transmitted to the base station via the FeedBack Information (FBI). Transmit diversity in these two modes can be seen as a beamforming method, where the beamforming weights at the base station are determined by each and every mobile user.
Closed loop transmit diversity schemes attempt to adjust the phases and amplitudes of the transmitted signals on the diversity antennas by using feedback information from each mobile user. Such strategy does not consider the overall system optimality, since the transmit diversity signals are radiated in the entire cell causing interference to other users. When transmit antennas are separated by more than half a wavelength, then grating lobes are generated. These grating lobes can be viewed as a form of a beam pointing error.
On the other hand, adaptive antenna systems are capable of steering the radiated energy towards (or from) the desired user, while at the same time minimizing the interference to other users, as reported in [1]. The major drawback of such strategy is the beam pointing error problem, which usually becomes more severe when the mobile users move from one fixed beam to the next.
Other known approaches involving beamforming and transmit diversity are described below.
A single antenna array employing open loop polarization transmit diversity on a single beam. A hybrid scheme consisting of a conventional beamformer (constructed using a single dual polarized antenna array) and open loop transmit diversity was studied in [4] for WCDMA systems. The two symbols generated from the space-time block coder are each transmitted on a different polarized antenna element. A conventional Bartlett (phase rotating) beamformer is used to direct the energy in the desired user's location.
A single antenna array employing open loop transmit diversity on two selected beams. In [5] a WCDMA link simulation involving a moving mobile and a fixed beam antenna system is presented. The base station decides from uplink measurements a set of two beams to transmit for the downlink. The base station then transmits the same symbols on different beams and in different time intervals. Equal amounts of power on each beam are used. Thus, the channel conditions are not matched, which implies that more power may be transmitted than necessary. If the angular spread of the mobile user is located within the beamwidth of the first downlink beam, then the power in the second beam will be degraded and at the same time additional interference to other users within the cell will be introduced.
An antenna array with antenna weights determined by feedback signalling with or without the aid of uplink measurements: In [6], downlink beamforming using a single antenna array forming two beams towards each mobile user is presented. Open loop transmit diversity signalling is used per beam. The antenna weights forming the first beam are determined by uplink measurements, while the antenna weights forming the second beam are computed by the mobile and transmitted to the base station. In [6], an alternative beamforming method is proposed, where a single beam is directed towards the mobile and the angle of departure of the beam is determined by the mobile user. In [7], an antenna array is proposed at the base station and the antenna weights are determined by the mobiles and transmitted to the base station by feedback signalling. Knowledge of the multi-element array structure for every base station is crucial. Thus, the base station antenna system design on every site must be known to every mobile user. Changes in the standards are required to accommodate these methods. If a mobile user dictates the antenna weights to be used for transmission, then the overall system optimality can be compromised.
A diversity antenna array with antenna weights determined by feedback signalling. Recently there have been proposals to the 3 GPP to standardize transmit diversity methods for four transmit antennas. One contribution has been the eigenbeamformer reported in [8], where the mobile user determines the optimal antenna weight by performing an eigendecomposition of the channel matrix. The optimal antenna weight vector is given by the principal eigenvector. In order to reduce the feedback rate, the eigenvectors corresponding to the two or more largest eigenvalues are fed back on a slow basis, i.e. the update of these vectors is done during several frames in WCDMA. On a fast level, e.g. slot by slot, the best weight at this particular fading pattern is chosen. Since the scheme is proposed to be mandatory in the terminal, each mobile terminal must implement the computationally expensive eigendecomposition algorithm. Since the feedback format must be standardized, this method is not adaptable to cover more or less antenna elements than the proposed four.
Diversity antenna systems with antenna arrays in each diversity branch. In [3] Orthogonal Transmit Diversity (OTD) and Space-Time Spreading (STS) are proposed to be used on each diversity branch for the CDMA2000 standard. Two groups of antenna arrays are separated far apart to ensure diversity. Beamforming is used on each group by placing the antenna elements within the group half a wavelength apart. No standard changes are required. In [7], diversity antenna systems are proposed for WCDMA that require modifications of the existing standards. Both [3] and [7] use distributed antenna arrays and the attendant addition of hardware complexity.
It is therefore desirable to avoid beam pointing errors without incurring disadvantages such as mentioned above.
According to the invention, the beam pointing error problem can be alleviated if data to a specific mobile is radiated using more than one downlink beam. The signal on each beam is also adjusted such that the received signals at each mobile can be coherently combined, thus minimizing the total transmit power while at the same time avoiding beam pointing errors. This is accomplished by combining beamforming and closed loop transmit diversity signalling.